Enhanced systems and methods for using a power converter for balancing modules in single-string and multi-string configurations

ABSTRACT

Apparatuses and methods include a solar array having one or more string buses of series-connected solar modules. The current outputs from the solar modules on each string bus can be balanced along with the voltage output from the string buses. Local management units coupled between the solar modules and the string buses are configured to control the voltage output from each solar module. When the solar modules on each string are balanced, and when the string buses are balanced (or before the solar modules and string buses are balanced), an inverter or other device connected to the solar array can find the array&#39;s maximum power point via a maximum power point tracking algorithm.

RELATED APPLICATIONS

The present application claims is a continuation application of U.S.patent application Ser. No. 12/612,641, filed Nov. 4, 2009 and entitled“Enhanced Systems and Methods for using a Power Converter for BalancingModules in Single-String and Multi-String Configurations”, which claimsthe benefit of the filing dates of Prov. U.S. App. Ser. No. 61/273,931,filed Aug. 10, 2009 and entitled “Enhanced Systems and Methods for usinga Power Converter for Balancing Panels in Single-String and Multi-StringConfigurations,” and Prov. U.S. App. Ser. No. 61/273,932, filed Aug. 10,2009, entitled, “Enhanced Systems and Methods for using a PowerConverter for Balancing Panels in Single-String and Multi-StringConfigurations,” the entire disclosures of which applications are herebyincorporated herein by reference.

The present application relates to U.S. Pat. No. 8,860,241, issued Oct.14, 2014 and entitled “Systems and Methods for using a Power Converterfor Transmission of Data over the Power Feed,” U.S. Pat. No. 7,602,080,issued Oct. 13, 2009 and entitled “Systems and Methods to Balance SolarPanels in a Multi-Panel System,” the entire disclosures of which arehereby incorporated herein by reference.

FIELD OF THE TECHNOLOGY

At least some embodiments of the disclosure relate to photovoltaicsystems in general, and more particularly but not limited to, improvingthe energy production performance of photovoltaic systems.

BACKGROUND

A traditional maximum power point tracking (MPPT) algorithm sees a solararray as if it were a single solar module (MPPT may pull and pushcurrent on all string buses and solar modules in a solar array in anequivalent fashion). As such, if solar modules in the solar arrayoperate at different working points on the I-V curve, due to differencesin installation, fabrication, or degradation over time, then an MPPTalgorithm may not be able to find the maximum power point (MPP) for thesolar array.

SUMMARY OF THE DESCRIPTION

Systems and methods in accordance with the present invention aredescribed herein. Some embodiments are summarized in this section.

In one embodiment, a solar array is described. The solar array maycomprise one or more string buses of series-connected solar modules.Each solar module may produce a current, and each string bus may producea voltage. The solar array may also comprise a controller. Thecontroller may be configured to balance the currents produced by thesolar modules in each string. The controller may also be configured tobalance the voltages produced by the strings in the solar array.

In another embodiment, a method is described comprising balancingcurrents produced by solar modules in each of one or more string busesof a solar array, and balancing voltages produced by the one or morestring buses of the solar array.

In another embodiment, a solar array is described. The solar array maycomprise a string of series-connected solar modules, each solar moduleproducing a current. The solar array may also include a controllerconfigured to balance the currents produced by the solar modules.

Other embodiments and features of the present invention will be apparentfrom the accompanying drawings and from the detailed description whichfollows.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments are illustrated by way of example and not limitation inthe figures of the accompanying drawings in which like referencesindicate similar elements.

FIGS. 1-3B illustrate local management units according to someembodiments.

FIG. 4A illustrates a photovoltaic system according to one embodiment.

FIG. 4B illustrates an embodiment of a solar array along with aninverter and a string combiner.

FIG. 5 illustrates a solar panel according to one embodiment.

FIGS. 6-8 show methods to improve performance of a photovoltaic systemaccording to some embodiments.

FIG. 9 illustrates a local management unit according to one embodiment.

FIG. 10A is a plot of carrier frequency for a local management unitaccording to one embodiment.

FIG. 10B illustrates a subsystem including a local management unitaccording to one embodiment.

FIG. 11A illustrates a photovoltaic system according to one embodiment.

FIG. 11B illustrates a receiver of a photovoltaic system according toone embodiment.

FIG. 12 illustrates a local management unit according to one embodiment.

FIGS. 13-18 illustrate operation of the local management unitillustrated in FIG. 12.

FIG. 19 illustrates a local management unit and transmission modulatoraccording to one embodiment.

FIG. 20 illustrates an exemplary inverter current controlled by amaximum power point tracking algorithm.

FIG. 21 illustrates exemplary solar module currents for strong and weaksolar modules.

FIG. 22 illustrates an exemplary composite I-V curve for solar modulesin a solar array.

FIG. 23 illustrates exemplary plots of current changes seen on twostring buses when connected in parallel.

FIG. 24 illustrates an exemplary current versus time diagram for astronger and weaker string bus when the voltage to the string busesfluctuates.

FIG. 25 illustrates an exemplary composite I-V curve for string buses ina solar array.

FIG. 26 illustrates an embodiment of a method of maximizing the poweroutput of a solar array by (1) balancing current outputs of solarmodules, (2) balancing voltage outputs of string buses, and (3) applyingan MPPT algorithm to the solar array.

FIG. 27 illustrates an I-V curve (2700) for a string bus where all solarmodules (2702) are operating at their ideal outputs.

FIG. 28 illustrates an I-V curve (2800) for a string bus where two solarmodules are operating as weak solar modules.

FIG. 29 illustrates an I-V curve (2900) for a string bus implementingthe systems and methods of this disclosure.

FIG. 30 illustrates an I-V curve (3000) for a solar array where allstring buses (3002) are operating at their ideal outputs.

FIG. 31 illustrates an I-V curve (3100) for a solar array where twostring buses are operating as weak string buses.

FIG. 32 illustrates an I-V curve (3200) for a solar array implementingthe systems and methods of this disclosure.

DETAILED DESCRIPTION

The following description and drawings are illustrative and are not tobe construed as limiting. Numerous specific details are described toprovide a thorough understanding. However, in certain instances, wellknown or conventional details are not described in order to avoidobscuring the description. References to one or an embodiment in thepresent disclosure are not necessarily references to the sameembodiment; and, such references mean at least one.

When solar modules are connected in series or mesh configuration, therecan be a problem in which weaker modules not only produce less energybut also affect other modules in the same string or wiring section. Bymeasuring one can determine that a few modules are weaker than theothers in most commercially installed strings. Thus, the string isgenerating less power than the sum available at each module if moduleswere operated separately.

At least one embodiment of the present disclosure provides methods andsystems to switch on and off weak modules in the string in a way thatthe current on the string bus from the good modules won't be affected bythe weak modules.

The present invention allows transmission of data from solar modules toa central (or system controller management) unit and other localmanagement units in an energy production or photovoltaic system withoutadding significant cost. One embodiment of the present inventioninvolves using the typically undesired electrical noise produced whenoperating local management units (sometimes referred to as “controllers”or “converters”) to act as a carrier system for data to be transferred.As there are a multitude of solar modules, each can be run on a slightlydifferent frequency. Such an approach allows a receiver in the energyproduction or photovoltaic system to identify the carrier signal of eachlocal management unit separately. This approach has the added benefit ofreducing the overall system noise, because each local management unitsends “noise energy” in a different part of the spectrum, thus helpingto avoid peaks.

FIGS. 1-3 illustrate local management units according to someembodiments. In FIGS. 1-3, local management units (101) are used toswitch on and off the solar module (102) periodically to improve theenergy production performance of the photovoltaic systems connected, atleast in part, in series. A local management unit may be variouslyreferred to as a solar module controller (or converter) or link moduleunit. One example of a local management unit is any of the various localmanagement units (solar module controllers) offered by Tigo Energy, Inc.of Los Gatos, Calif.

In FIG. 1, a management unit (101) is local to the solar module (102)and can be used to periodically couple the solar module (102) to theserial power bus (103) via the switch Q1 (106), to improve the totalpower output for the string of solar modules connected to the serialpower bus in series.

The local management unit (LMU) (101) may include a solar modulecontroller to control the operation of the solar module (102) and/or alink module unit to provide connectivity to the serial power bus (103)for energy delivery and/or for data communications.

In one embodiment, the command to control the operation of the switch Q1(106) is sent to the local management unit (101) over the photovoltaic(PV) string bus (power line) (103). Alternatively, separate networkconnections can be used to transmit the data and/or commands to/from thelocal management unit (101).

In FIGS. 1 and 2, the inputs (104 a, 104 b, 104 c) to the localmanagement unit (101) are illustrated separately. However, the inputs(104 a, 104 b, 104 c) are not necessarily communicated to localmanagement unit (101) via separate connections. In one embodiment, theinputs are received in the local management unit via the serial powerbus (103).

In FIG. 1, the solar module (102) is connected in parallel to thecapacitor C1 (105) of the local management unit (101). The diode D1(107) of the local management unit (101) is connected in series in theserial power bus (103) which may or may not be part of an overall meshconfiguration of solar modules. The switch Q1 (106) of the localmanagement unit can selectively connect or disconnect the solar module(102) and the capacitor C1 (105) from a parallel connection with thediode D1 (107) and thus connect or disconnect the solar module (102)from the serial power bus (103).

In FIG. 1, a controller (109) of the local management unit (101)controls the operation of the switch (106) according to the parameters,such as duty cycle (104 a), phase (104 b) and synchronization pulse (104c).

In one embodiment, the controller (109) receives the parameters (104 a,104 b, 104 c) from a remote management unit via the serial power bus(103) or a separate data communication connection (e.g., a separate databus or a wireless connection). In some embodiment, the controller (109)may communicate with other local management units connected on theserial power bus (103) to obtain operating parameters of the solarmodules attached to the serial power bus (103) and thus compute theparameters (e.g., 104 a and 104 b) based on the received operatingparameters. In some embodiment, the controller (109) may determine theparameter (e.g., 104 a and 104 b) based on the operating parameters ofthe solar module (102) and/or measurements obtained by the controller(109), without communicating with other local management units of othersolar modules, or a remote system management unit.

In FIG. 2, a system (100) has a local management unit (101) coupled tothe solar module (102). The local management unit (101) is connectedbetween the solar module (102) and the string bus (103) to improve thetotal power output for the whole string on the serial power bus (103).Commands to the local management unit (101) can be sent over thephotovoltaic (PV) string bus (power line) (103). To make the figure moreclear, the inputs (104 a, 104 b, 104 c) to the controller (109) of thelocal management unit (101) were drawn separately, which does notnecessarily indicate that the inputs (104 a, 104 b, 104 c) are providedvia separate connections and/or from outside the local management unit(101). For example, in some embodiments, the controller (109) maycompute the parameters (104 a, 104 b, 104 c) based on measurementsobtained at the local management unit (101), with or without datacommunications over the serial power bus (103) (or a separate datacommunication connection with other management units).

In FIG. 2, the local management unit (101) is connected in one side tothe solar module (102) in parallel and on the other side in series to astring of other modules, which may or may not be part of an overall meshconfiguration. The local management unit (101) may receive, amongothers, three inputs or types of input data, including a) requested dutycycle (104 a), which can be expressed as a percentage (e.g., from 0 to100%) of time the solar module (102) is to be connected to the serialpower bus (103) via the switch Q1 (106), b) a phase shift (104 b) indegrees (e.g., from 0 degree to 180 degree) and c) a timing orsynchronization pulse (104 c). These inputs (e.g., 104 a, 104 b and 104c) can be supplied as discrete signals, or can be supplied as data on anetwork, or composite signals sent through the power lines orwirelessly, and in yet other cases, as a combination of any of theseinput types.

In FIG. 2, the local management unit (101) periodically connects anddisconnects the solar module (102) to and from the string that forms theserial power bus (103). The duty cycle (104 a) and the phase (104 b) ofthe operation of the switch Q1 (106) can be computed in a number of waysto improve the performance of the system, which will be discussedfurther below.

In FIG. 2, the local management unit (101) includes a capacitor C1 (105)and a switch Q1 (106), as well as a diode D1 (107). In FIG. 2, the diodeD1 (107) is supplemented with an additional switch Q2 (108), which actsas a synchronous rectifier to increase efficiency. In one embodiment,the additional switch Q2 (108) is open (turned off) when the switch Q1(106) is closed (turned on) to attach the solar module (102) (and thecapacitor C1 (105)) to the serial power bus (103).

In some cases, a filter (not shown), including a serial coil and aparallel capacitor, is also used. The filter may be placed at the localmanagement unit or placed just before the fuse box or inverter, or bepart of either one of those.

In FIG. 2, the controller (109) is used to process the input signals(e.g., 104 a, 104 b, 104 c) and drive the switches Q1 (106) and Q2(108). In one embodiment, the controller (109) is a small single chipmicro controller (SCMC). For example, the controller (109) may beimplemented using Application-Specific Integrated Circuit (ASIC) orField-Programmable Gate Array (FPGA). The controller (109) can even beimplemented in discrete, functionally equivalent circuitry, or in othercases a combination of SCMC and discrete circuitry. It will be generallyreferred to as single chip micro controller (SCMC) herein, but anyimplementation may be used.

In one embodiment, the controller (109) is coupled to the solar module(102) in parallel to obtain power for processing; and the controller(109) is coupled to the serial power bus (103) to obtain signalstransmitted from other management units coupled to the serial power bus(103).

By switching the module (102) (or groups of cells, or a cell) on and offto the string periodically, the local management unit (101) may lowerthe voltage reflected to the string bus (103) (e.g., a lower averagevoltage contributed to the string bus) and can cause the currentreflected to the string bus (103) to be higher, nearer the level itwould be if the module was not weak, generating a higher total poweroutput.

In one embodiment, it is preferable to use different phases to operatethe switches in different local management units on a string to minimizevoltage variance on the string.

In FIG. 3A, the local management unit (101) provides two connectors (112and 114) for serial connections with other local management unit (101)to form a serial power bus (103). The controller (109) controls thestates of the switches Q1 (106) and Q2 (108).

In FIG. 3A, when the controller (109) turns on the switch (106), thepanel voltage and the capacitor C1 (105) are connected in parallel tothe connectors (112 and 114). The output voltage between the connectors(112 and 114) is substantially the same as the output panel voltage.

In FIG. 3A, during the period the switch (106) is turned off (open), thecontroller (109) turns on (closes) the switch (108) to provide a patharound the diode D1 (107) to improve efficiency.

In FIG. 3A, when the switch (106) is turned off (open), the panelvoltage charges the capacitor C1 (105), such that when the switch (106)is turned on, both the solar panel and the capacitor (105) providescurrents going through the connectors (112 and 114), allowing a currentlarger than the current of the solar panel to flow in the string (theserial power bus (103)). When the switch (106) is turned off (open), thediode D1 (107) also provides a path between the connectors (112 and 114)to sustain the current in the string, even if the switch (108) is offfor some reasons.

In one embodiment, the controller (109) is connected (not shown in FIG.3A) to the panel voltage to obtain the power for controlling theswitches Q1 (106) and Q2 (108). In one embodiment, the controller (109)is further connected (not shown in FIG. 3A) to at least one of theconnectors to transmit and/or receive information from the string. Inone embodiment, the controller (109) includes sensors (not shown in FIG.3A) to measure operating parameters of the solar panel, such as panelvoltage, panel current, temperature, light intensity, etc.

FIG. 3B shows an alternative three terminal implementation of the localmanagement unit 101 shown in FIG. 3A. In FIG. 3B, a panel voltage (180)is connected to terminals (182, 184). Terminals (182, 186) are connectedto the string bus (103). A module driver (110) and a single chip microcontroller (SCMC) control the switches Q1 and Q2. Under normal operatingconditions, Q1 is on to allow normal operation of the system. Whenstring current exceeds source capability, and as a result source voltagedrops, Q1 and Q2 start a PWM (pulse width modulation) operation undercontrol of the module driver (110). PWM involves modulation of dutycycle to control the amount of power sent to the load. This allowsstring current to remain constant, and input voltages can be maintainedat the maximum power point. This implementation protects transistorsduring low voltage or short situations. In one embodiment, a single chipmicro controller (SCMC) (109) can be connected in parallel to the diodeD1 (107) to function in the manner of the SCMC 109 as described above.In one embodiment, the module driver (110) and the single chip microcontroller (SCMC) (109) can be integrated in a single controller asshown in, for example, FIG. 3A. As discussed above, single chip microcontroller (SCMC) (109) can receive the inputs (104 a, 104 b, 104 c). Asshown in FIG. 3B, in one embodiment, the inputs (104 a, 104 b, 104 c)are provided with a communications interface (not shown) coupled to amaster controller (not shown). In one embodiment, other inputs (104 n)constituting information about other operating parameters can also becommunicated to the single chip micro controller (SCMC) (109) from thecommunications interface. In one embodiment, the other inputs (104 n)can be information that is communicated bi-directionally. As discussedabove, the power supply connections in the figures, including FIG. 3B,are not necessarily shown for purposes of clarity and so as not toobscure the invention.

FIG. 4A illustrates a photovoltaic system (200) according to oneembodiment. In FIG. 4A, the photovoltaic system 200 is built from a fewcomponents, including photovoltaic modules (201 a, 201 b, . . . , 201n), local management unit units (202 a, 202 b, . . . , 202 n), aninverter (203), and a system management unit (204).

In one embodiment, the system management unit (204) is part of theinverter (203), the combiner box (206), a local management unit, or astand-alone unit. The solar modules (201 a, 201 b, . . . , 201 n) areconnected in parallel to the local management units (202 a, 202 b, . . ., 202 n) respectively, which are connected in series to form a stringbus (205), which eventually is connected to an inverter (203) and themanagement unit (204). The solar module (201 a), for example, isconnected to the local management unit (202 a) by the terminals (182,184, 186) (FIG. 3B). As shown in FIG. 4A, in one embodiment, theterminal (182), which connects to the panel voltage and the stringvoltage, is connected to the depicted left connection between the solarmodule (201 a) and the local management unit (202 a) and connected tothe depicted left connection between the local management unit (202 a)and the string bus (205). The terminal (184), which is connected to thepanel voltage, is connected to the depicted right connection between thebetween the solar module (201 a) and the local management unit (202 a).The terminal (186), which is connected to the string voltage, isconnected to the depicted right connection between the local managementunit (202 a) and the string bus (205).

In FIG. 4A, the string bus (205) can be connected to the inverter (203)directly or as part of a mesh network or combiner boxes or fuse boxes(not shown). An isolated local management unit can be used as a combinerbox (206) to adjust all voltages before connecting to the inverter(206); or, a single or multi-string inverter can be used. To limit thechanges in the voltage of the bus, the management unit (204) may assigna different phase for each of the local management units (202 a, 202 b,202 n). In one embodiment, at any given time, a maximum of apredetermined number of solar modules (e.g., one single solar module)are disconnected from the string bus (205).

In one embodiment, beyond the module connection the local managementunits can have the signal inputs, including but not limited to dutycycle (104 a), phase (104 b) and synchronization pulse (104 c) (e.g., tokeep the local management units synchronized). In one embodiment, thephase (104 b) and the synchronization pulse (104 c) are used to furtherimprove performance, but the local management unit (101) can workwithout them.

In one embodiment, the local management unit may provide output signals.For example, the local management unit (101) may measure current andvoltage at the module side and optionally measure current and voltage inthe string side. The local management unit (101) may provide othersuitable signals, including but not limited to measurements of light,temperature (both ambient and module), etc.

In one embodiment, the output signals from the local management unit(101) are transmitted over the power line (e.g., via power linecommunication (PLC)), or transmitted wirelessly.

In one embodiment, the system management unit (204) receives sensorinputs from light sensor(s), temperature sensor(s), one or more each forambient, solar module or both, to control the photovoltaic system (200).In one embodiment, the signals may also include synchronization signals.For example, a management unit can send synchronization signalsperiodically to set the timing values, etc.

Using the described methods the local management unit can be a verynon-expensive and reliable device that can easily increase thethroughput of a photovoltaic solar system by a few (e.g., signal or lowdouble digits) percentage points. These varied controls also allowinstallers using this kind of system to control the VOC (open circuitvoltage) by, for example by shutting off some or all modules. Forexample, by using the local management units of the system, a fewmodules can be disconnected from a string if a string is getting to theregulatory voltage limit, thus more modules can be installed in astring.

In some embodiments, local management units can also be used within thesolar panel to control the connection of solar cells attached to stringsof cells within the solar panel.

FIG. 5 illustrates a solar panel according to one embodiment. In oneembodiment, the solar panel (300) has a few strings of solar cells(e.g., three solar cell strings per module). In FIG. 5, a localmanagement unit (101) can be applied to a group of cells (301) within astring of an individual solar panel (300), or in some cases to each cell(301) in a solar panel (300).

In FIG. 5, a group of solar cells (301) that are attached to a localmanagement unit (101) may be connected to each other in series, inparallel, or in a mesh configure. A number of local management units(101) connect the groups of the solar cells (301) in a string to provideoutput for the solar panel (300).

Some embodiments of the disclosure include methods to determine the dutycycles and/or phases for local management units connected to a string ormesh of solar modules.

In some embodiments, the duty cycle of all local management units in astring or mesh can be changed, to increase or decrease the stringvoltage. The duty cycles may be adjusted to avoid exceeding the maximumvoltage allowed. For example, the maximum voltage may be limited by thecombiner box (206), the inverter (203), or any other load connected tothe string bus (205), or limited by any regulations applicable to thatsystem. In some embodiments, the duty cycles are adjusted to align thevoltage of multiple strings.

In some embodiments, the duty cycle of one local management unit (101)in a string can be changed to cause higher current in that localmanagement unit (101) and overall higher power harvesting.

In one embodiment, the duty cycles are computed for the solar modulesthat are connected to a string via the corresponding local managementunits. The duty cycles can be calculated based on the measured currentand voltages of the solar modules and/or the temperatures.

After an initial set of duty cycles is applied to the solar modules, theduty cycles can be further fine tuned and/or re-adjusted to changes,such as shifting shading etc., one step a time, to improve powerperformance (e.g., to increase power output, to increase voltage, toincrease current, etc.). In one embodiment, target voltages are computedfor the solar modules, and the duty cycles are adjusted to drive themodule voltage towards the target voltages.

The methods to compute the duty cycles of the solar modules can also beused to compute the duty cycles of the groups of solar cells within asolar module.

FIGS. 6-8 show methods to improve performance of a photovoltaic systemaccording to some embodiments.

In FIG. 6, at least one operating parameter of a solar energy productionunit coupled to a string via a management unit is received (401) andused to identify (403) a duty cycle for the management unit to connectthe solar energy production unit to string. The solar energy productionunit may be a solar module, a group of solar cells within a solarmodule, or a single solar cell in a string in a solar module. The dutycycle is adjusted (405) to optimize the performance of the solar energyproduction unit and/or the string.

For example, the duty cycle can be adjusted to increase the current inthe string and/or the solar energy production unit, to increase theoutput power of the string and/or the solar energy production unit, toincrease the voltage of the solar energy production unit, etc.

In FIG. 7, the operating voltages of a plurality of solar panelsconnected in series are received (421) and used to identify (423) asecond solar panel having the highest operating voltage (highest outputpower) in the string.

In FIG. 7, a duty cycle of a first solar panel is computed (425) basedon a ratio in operating voltage between the first and second solarpanels. Alternatively, the duty cycle can be computed based on a ratioin output power between the first and second solar panels.Alternatively, the duty cycle can be computed based on a ratio betweenthe first and second solar panels in estimated/computed maximum powerpoint voltage. Alternatively, the duty cycle can be computed based on aratio between the first and second solar panels in estimated/computedmaximum power point power.

The duty cycle of the first solar panel is adjusted (427) to improve theperformance of the first solar energy production unit and/or the string,until a decrease in the operating voltage of the second solar panel isdetected. For example, the duty cycle of the first solar panel can beadjusted to increase the total output power of the string, to increasethe current of the string, to increase the current of the first solarpanel, to drive the voltage of the first solar panel towards a targetvoltage, such as its maximum power point voltage estimated based on itscurrent operating parameters, such as temperature or a voltagecalculated using its estimated maximum power point voltage.

In FIG. 7, in response to the detected decrease in the operating voltageof the second solar panel which had the highest operating voltage, theadjustment in the duty cycle of the first solar panel that causes thedecrease is undone/reversed (429).

In FIG. 7, the duty cycle of the second solar panel is optionallydecreased (431) to increase the operating voltage of the second solarpanel. In some embodiments, the strongest solar panel (or strong panelswithin a threshold from the strongest panel) is not switched off line(e.g., to have a predetermined duty cycle of 100%).

In one embodiment, the duty cycle of the second solar panel isrepeatedly decreased (429) until it is determined (431) that thedecrease (429) in the duty cycle of the second solar panel cannotincrease the voltage of the second solar panel.

In FIG. 8, operating parameters of a plurality of solar panels connectedin a string are received (441) and used to identify (443) a firstmaximum power point voltage of a first solar panel. A second solar panelhaving the highest operating voltage (or output power) in the string isidentified. A second maximum power point voltage of the second solarpanel is identified (447) based on the received operating parameters andused to compute (449) a target voltage for the first solar energyproduction unit. In one embodiment, the target voltage is a function ofthe first and second maximum power point voltages and the highestoperating voltage identified (445) in the second solar panel in thestring. The duty cycle of the first solar energy production unit isadjusted to drive the operating voltage of the first solar panel towardsthe target voltage.

Alternatively, the target voltage may be set as the first maximum powerpoint voltage of the first solar panel.

In one embodiment, to adjust voltage a same factor is applied to allmodules in that string. For example, in a case of a first module A1 thatis producing only 80%, and the voltage of the whole string needs to be5% lower, the duty cycle of A1 is 80% multiplied the duty cycle appliedto the whole string (which is Y in this example) so module A1 then hasY×0.8 as duty cycle.

In some embodiments, the system management unit (204) and/or the localmanagement units (e.g., 202 a, 202 b, . . . , 202 n) are used solely orin combination to determine the parameters to control the operations ofthe switches.

For example, in one embodiment, a system management unit (204) is the“brain” of the system, which decides on the duty cycle and phaseparameters.

For example, in another embodiment, each local management unitbroadcasts information to the other local management units on the stringto allow the individual local management units to decide their own dutycycle and phase parameters.

In some embodiment, a local management unit may instruct one or moreother local management units to adjust duty cycle and phase parameters.For example, the local management units on a string bus (205) may electone local management unit to compute the duty cycle and phase parametersfor other local management units on the string.

For example, in some embodiment, the system management unit (204) maydetermine one or more global parameters (e.g., a global duty cycle, themaximum power on the string, the maximum voltage on the string, etc.),based on which individual local management units adjust their own dutycycles.

In some embodiments, a local management unit may effectively self manageand determine its own duty cycles without relying upon communicatingwith other management units. For example, the local management unit mayadjust its duty cycle for connecting its solar module to the string tooperate the solar module at the maximum power point. No local managementunit is in control over the system, and each adjusts its own duty cycle(and thus, its power and voltage.)

In one embodiment, module voltages are measured by the local managementunits in the same string at substantially/approximately the same timeand used to identify the strongest solar module. A strongest solarmodule provides the most power in the string. Since the modules areconnected in series, the solar module having the highest module voltagein the string can be identified as the strongest solar module. In someembodiment, the operating voltage and current of the solar module aremeasured to determine the power of the solar module.

Additional approaches can be implemented to control the voltage, poweroutput, or the efficiency of one or more strings of solar modulecontrollers as described above. In some embodiments, a system controllermanagement unit controls the operation of a plurality of localmanagement units in one or more strings. In some embodiments, one ormore local management units controls the operation of a plurality oflocal management units in one or more strings. In some embodiments, thelocal management unit may only control its own operation, or may controlthe operation of itself and other local management units in the samestring.

One or more local management units in a string may have the capabilityto control the operation of other local management units in the samestring. In one embodiment, a single local management unit can beselected to be a controlling local management unit to control aplurality of panels in a string. The controlling local management unitin a string can be selected using any suitable protocol. In oneembodiment, in a string of local management units, the first localmanagement unit that announces its intent to take control of othermodules in the string could become the controlling local managementunit.

In one embodiment, to improve power output by a string, one or morelocal management units can each receive module voltage from all localmanagement units in the same string and identify the strongest localmanagement unit (i.e., the one with the maximum power and voltage). Eachlocal management unit can then set its own duty cycle as a function ofthe received voltage.

In one embodiment, after the highest module voltage Vm in the string isidentified, the duty cycle for each module can be computed as a functionof a ratio between the module voltage V of the module and the highestmodule voltage Vm. For example, the duty cycle for a module can becomputed as 1−((Vm−V)/Vm)=V/Vm. In one embodiment, a particular localmanagement unit receives the voltages of all other local managementunits at the same time or substantially same time (e.g., all voltagesare received within an interval of less than one second.)

In one embodiment, the system management unit (204) may identify thehighest module voltage from the module voltages received from the localmanagement units (202 a, 202 b, . . . , 202 n), and compute the dutycycles for the corresponding local management units (202 a, 202 b, . . ., 202 n).

In one embodiment, the local management units (202 a, 202 b, . . . , 202n) may report their module voltages on the string bus (205) to allowindividual local management units (202 a, 202 b, . . . , 202 n) toidentify the highest module voltage and compute the duty cycles, withoutrelying upon the system management unit (204).

In one embodiment, one of the local management units (202 a, 202 b, 202n) may identify the highest module voltage and/or compute the dutycycles for the other local management units (202 a, 202 b, . . . , 202n).

In one embodiment, the duty cycles are determined and/or adjustedperiodically (e.g., every 30 seconds). The intervals can take intoaccount various environmental factors (e.g., where shadows on a solarpanel are cast on different parts of the panel over the course of aday).

In one embodiment, after the duty cycles for the solar modules on thestring are set based on the module voltage ratio relative to the highestmodule voltage in the string, the duty cycles can be fine tuned toincrease the power performance. The duty cycles can be fine tuned onestep a time, until a decrease of voltage of the module with the highestpower is detected. In response to the detected decrease, the last changethat caused the decrease can be reversed (undone). The fine tuning ofthe duty cycles can be used to reach the peak performance point (e.g.,for maximum power point tracking).

In one embodiment, after the strongest module is identified, the dutycycles of the solar modules on the string are adjusted until the modulewith the highest power in the string decrease its voltage. Sincedecreasing the duty cycle of a solar module decreases the time periodthe module is connected to the string and thus increases its voltage,the duty cycle of the module with the highest power in the string can bedecreased to increase its voltage, in response to the decrease in itsvoltage caused by the adjustment to the duty cycles of other solarmodules on the string. For example, the duty cycle of the module withthe highest power in the string can be decreased until its voltage ismaximized.

The performance of solar modules may vary significantly withtemperature. A system capable of measuring temperature can implementmethods for controlling the voltage, power output, or the efficiency ofone or more strings of solar module controllers using module temperatureas a factor. In one embodiment, the local management unit measuresmodule and ambient temperatures for some methods to determine the dutycycles. For example, the operating parameters measured at the localmanagement units (e.g., 202 a, 202 b, . . . , 202 n), such as moduletemperature, can be used to compute the estimated voltages of the solarmodules at their maximum power points. For example, a formula presentedby Nalin K. Gautam and N. D. Kaushika in “An efficient algorithm tosimulate the electrical performance of solar photovoltaic arrays,”Energy, Volume 27, Issue 4, April 2002, pages 347-261, can be used tocompute the voltage Vmp of a solar module at the maximum power point.Other formulae can also be used. Once the maximum power point voltageVmp of a solar module is computed or estimated, the duty cycle of thesolar module connected to a string can be adjusted to drive the modulevoltage to the computed/estimated maximum power point voltage Vmp, sincedecreasing the duty cycle of a solar module normally increases itsvoltage.

In one embodiment, a local management unit may adjust the duty cycle ofthe solar module connected to the local management unit to change themodule voltage to the computed/estimated maximum power point voltageVmp, without having to communicate with other management units.

In one embodiment, a local management unit (or a system management unit)may adjust the duty cycle of the solar module connected to the localmanagement unit to perform maximum power point tracking.

In one embodiment, after identifying the strongest module andcomputing/estimating the maximum power point voltage Vmpm of thestrongest module, the duty cycle for each module on a string can becomputed as a function of a ratio between the maximum power pointvoltage Vmp of the module and the maximum power point voltage Vmpm ofthe strongest module. For example, the duty cycle for a module can becomputed as 1−((Vmpm−Vmp)/Vmpm)=Vmp/Vmpm. The duty cycle can beperiodically updated, based on the current operating parametersmeasured, and/or fine tuned until a decrease in the voltage of thestrongest module is detected.

Alternatively, a target voltage for each module on the string can becomputed as a function of a ratio between the maximum power pointvoltage Vmp of the module and the maximum power point voltage Vmpm ofthe strongest module. For example, the target voltage for a module canbe computed as Vm×Vmp/Vmpm, where Vm is the measured voltage of thestrongest module. The duty cycle of the module can be changed to drivethe module voltage of the module towards the target voltage.

In one embodiment, after identifying the strongest module andcomputing/estimating the maximum power point power Pmpm of the strongestmodule, the duty cycle for each module on a string can be computed as afunction of a ratio between the maximum power point power Pmp of themodule and the maximum power point power Pmpm of the strongest module.For example, the duty cycle for a module can be computed as1−((Pmpm−Pmp)/Pmpm)=Pmp/Pmpm. The duty cycle can be periodicallyupdated, based on the current operating parameters measured, and/or finetuned until a decrease in the voltage of the strongest module isdetected, since decreasing the duty cycle normally increases the modulevoltage.

In one embodiment, a target voltage for each module on the string can becomputed as a function of a ratio between the maximum power point powerPmp of the module and the maximum power point power Pmpm of thestrongest module. For example, the target voltage for a module can becomputed as Vm×Pmp/Pmpm, where Vm is the measured voltage of thestrongest module. The duty cycle of the module can be changed to drivethe module voltage of the module towards the target voltage, sincedecreasing the duty cycle normally increases the module voltage.

In one embodiment, the duty cycle for each local management unit ischanged to increase the current of the solar module attached to thelocal management unit (e.g., based on the measurement of the voltage andcurrent of the solar module), until the maximum current is achieved.This method assumes that string maximum power can be achieved with someaccuracy by driving each local management unit to maximum current. Inone embodiment, the voltages and currents of the solar modules aremeasured for tuning the duty cycles for maximum power point tracking forthe string. The measurements of the voltages and currents of the solarmodules also enable the local management units to additionally serve asa module level monitoring system.

The duty cycles can be adjusted by the system management unit (e.g.,204) based on the measurements reported by the local management units(e.g., 202 a, 202 b, . . . , 202 n), or adjusted directly by thecorresponding local management units (e.g., 202 a, 202 b, . . . , 202n).

In one embodiment, during the process of setting and/or tuning the dutycycles, the maximum power point tracking operation by the inverter (203)is frozen (temporarily stopped). Light intensity at the solar modules ismonitored for changes. When the light intensity at the solar modulesstabilizes, the voltage and current of the solar modules are measuredfor the determination of the duty cycles. Then normal operation resumes(e.g., unfreezing of maximum power point tracking operation).

In one embodiment, the local management units measure the voltages andcurrents of the solar modules to determine the power of the solarmodules. After identifying the highest power Pm of the solar module onthe string, the duty cycles of the solar modules on the string aredetermined by the power ratio relative to the highest power Pm. Forexample, if a module produces 20 percent less power, it will bedisconnected from the string bus about 20 percent of the time. Forexample, if a module produces power P, its duty cycle can be set to1−((Pm−P)/Pm)=P/Pm.

In one embodiment, a predetermined threshold is used to select the weakmodules to apply duty cycles. For example, in one embodiment, when amodule produces power less than a predetermine percent of highest powerPm, a duty cycle is calculated and applied to the solar module. If themodule is above the threshold, the module is not disconnected (and thushaving a duty cycle of 100%). The threshold may be based on the power,or based on the module voltage.

In one embodiment, the system management unit (204) finds the dutycycles for the local management units (202 a, 202 b, . . . , 202 n) andtransmits data and/or signals representing the duty cycles to the localmanagement units (202 a, 202 b, . . . , 202 n) via wires or wirelessconnections. Alternatively, the local management units (202 a, 202 b, .. . , 202 n) may communicate with each other to obtain the parameters tocalculate the duty cycles.

In one embodiment, the system management unit (204) knows all thedifferent duty cycles indicated for the local management units (202 a,202 b, . . . , 202 n).

In one embodiment, during power fine tuning, the system management unit(204) sends the appropriate data/signal to the appropriate localmanagement units (202 a, 202 b, . . . , 202 n), and then the systemmanagement unit (204) calculates the total power of the string andcorrects the duty cycle to produce maximum power. Once maximum power isachieved, the duty cycles for the local management units (202 a, 202 b,. . . , 202 n) may be saved in a database and serve as a starting pointfor the corresponding local management units (202 a, 202 b, . . . , 202n) at the same time of day on the next day. Alternatively, a localmanagement may store the duty cycle in its memory for the next day.

The stored duty cycles can be used when there is a fixed shade on themodules, such as a chimney, a tree, etc., which will be the same shadeon any day at the same time. Alternatively, historical data may not besaved, but may be recalculated from scratch on each run, for exampleevery 30 minutes.

In one embodiment, the light intensity at the solar modules is monitoredfor changes. The duty cycles are calculated when the light intensitydoes not change significantly. If there are changes in sun lightradiation at the solar modules, the system will wait until theenvironment stabilizes before applying or adjusting the duty cycles.

In one embodiment, the system management unit (204) can communicate withthe inverter as well. When the environment is not stable (e.g., when thesun light radiation is changing), the inverter may stop maximum powerpoint tracking. In such a situation, the inverter can be set up for itsload, instead of tracking for maximum power point. Instead of using theinverter to perform maximum power point tracking, the system managementunit (204) and the local management units (202 a, 202 b, . . . , 202 n)are used to set the operating parameters and balance the string.

Alternatively, when the environment is not stable but measurements andcalculation are done faster than the MPPT is working, there may be noneed to stop the MPPT on the inverter. Alternatively, when theenvironment is not stable, measurements can be taken few times for thesame radiation until a stable result is achieved.

Many variations may be applied to the systems and methods, withoutdeparting from the spirit of the invention. For example, additionalcomponents may be added, or components may be replaced. For example,rather than using a capacitor as primary energy store, an inductor maybe used, or a combination of inductor and capacitor. Also, the balancebetween hardware and firmware in the micro controllers or processors canbe changed, without departing from the spirit of the invention. In somecases, only some problematic modules may have a local management unit,for example in a shaded or partially shaded or otherwise differentsituation. In yet other cases, local management units of strong modulesmay be virtually shut off. The methods for determining the duty cyclesfor the solar modules can also be used to determine the duty cycles ofgroups of cells connected via local management units in a string withina solar panel/module.

FIG. 9 shows an overview of a local management unit (202 x) that ismodified from the local management unit (101) discussed above inrelation to FIG. 3B. In FIG. 9, local management unit (202 x) contains asingle chip micro controller (SCMC) (109). In one embodiment, all of thefeatures and details of the local management units discussed above applyto the local management unit (202 x) and are not repeated for purposesof clarity. In one embodiment, some of the features and details of thelocal management units discussed above selectively apply to the localmanagement unit (202 x) and are not repeated for purposes of clarity.The module driver (110) is connected in parallel with the capacitor C1,and is also connected between the switches Q1 and Q2. The microcontroller (109) contains various operating parameters regarding thelocal management unit (202 x), such as the voltage, current, etc. Themicro controller (109) can run suitably programmed software (120 a-n) tomodulate the chopping frequency of the switches Q1 and Q2. The switchesQ1 and Q2 perform a duty cycle according to the formula calculated aspreviously described. A duty cycle would result in minor variations fromcycle to cycle (i.e., in the inter cycle) that can be used to encodeusing MFM (modified frequency modulation), Manchester-type encoding, orother suitable time-delay type encoding technique with or withoutadditional error correction. As discussed further below, the approach ofmodulating, for example, the PWM inter cycle would allow a receiver(301) at the end of the string bus (205) to measure the differentvariations of each of the local management units. Also, the localmanagement units each can have a slightly different base frequency sothat their respective harmonics would not cover each other, althoughthey would move in a similar range. This approach has the added benefitof reducing overall EMI of the system.

FIG. 10A is a plot of the upper half of a frequency spectrum (500) of acarrier frequency (501) for a particular local management unit. Thefrequency spectrum (500) shows the harmonics fn1-fnn as elements (505a-n). Arrows above the harmonics fn1-fnn (505 a-n) indicate they wobblearound with the variations in pulse width modulation from cycle tocycle. Also shown is a notch filter curve (504), which can be used toremove significant noise to avoid EMI problems in the system and tocomply with FCC and other regulatory agency regulations as needed.

FIG. 10B shows an overview of a subsystem (510) that includes the localmanagement unit (202 x), the panel voltage (180), terminals (182, 184,186), and a notch filter (506). In one embodiment, the notch filter(506) includes an inductor Ln and a capacitor Cn. The notch filter (506)acts as a low pass filter and relies on the internal capacity of thesingle chip micro controller (SCMC) of the local management unit (202x). A notch frequency of the notch filter (506) sits on the switchingfrequency to suppress noise. In one embodiment, additional or differentfilters may be used.

FIG. 11A shows an overview of a system (200) with a string bus (205)similar to that of system (200) previously discussed in relation to FIG.4A. In FIG. 11A, a receiver subsystem (300) is a receiving portion of amodem associated with a head end to receive modulated signals from localmanagement units, as described in more detail below. The receiversubsystem (300) includes a receiving path separate from the string bus(205) and the combiner box (206) so that the modulated signals from thelocal management units can be recovered before provision to the combinerbox (206) and significant noise therein. The receiver subsystem (300)includes a receiver (301), a sensing line (302), and a data output line(303). The sensing line (302) is connected to the string bus (205) andthe data output line (303) connects to the combiner box (206). In oneembodiment, the subsystem (300) can be inside the inverter (203). In oneembodiment, the subsystem (300) is contained in the combiner box (206).The subsystem (300) is shown external to the combiner box (206) in FIG.11A for purposes of clarity.

FIG. 11B shows the receiver (301). The receiver (301) includes a bandpass filter (310), a mixer (311), a beat oscillator (VCO) (312), amultiband pass filter (313), a microcontroller (314), and a power supply(315). Data from the local management unit arrives over the power bus205 via sensing line (302), and then passes through the band pass filter(310) to improve signal-to-noise ratio. The mixer (311) mixes the outputof the band pass filter (310) and the output of the VCO (312). Theoutput of the mixer (311) is then applied to the multiband pass filter(313), where the signal is analyzed in multiple band, frequency, andtime domains. The output of the multiband pass filter (313) is analyzedby the microcontroller (314). The power supply (315) can receive powerfrom the string bus (205) or from the inverter (203) and provide it tothe various elements of the receiver (301).

In one embodiment, the receiver (301) can manage communications from allthe local management units. In one embodiment, each local managementunit can have its own receiver. In one embodiment, a receiver can beimplemented in hardware (HW) only. In one embodiment, a digital radiocan be used as the receiver, in which case an analog to digitalconverter (ADC) samples the signals and all the processing is done in amicrocontroller or a digital signal processor using software (SW), orany combination of SW and HW.

FIG. 12 shows a novel topology of a local management unit (1200) as adistributed converter and remaining aspects of the local management unit(1200), as discussed above, are not shown for purposes of clarity. Inthe energy production or photovoltaic system, the local management unit(1200) in FIG. 12 can be used alternatively to the local managementunits discussed above. The local management unit (1200) is aseries-resonant converter with phase shift operation for light loadoperation. The local management unit (1200) includes capacitor Cin,switches Q1, Q2, Q3, Q4, inductor LR, capacitor CR, transformer having aprimary winding Tp coupled to a secondary winding Ts, diodes D1, D2, andtwo capacitors Cout. A typical range of input voltage Vin for the localmanagement unit (1200) is the standard panel voltage of Vmp plus orminus 20%. Output voltage Vout of the distributed converter is a fixedvalue of 375V plus or minus a few percentage points.

In operation, switch Q1 and switch Q2 are controlled oppositely, andswitch Q3 and switch Q4 are controlled oppositely. When switch Q1 is on,switch Q3 is on. When switch Q2 is on, switch Q4 is on. The current canbe increased or decreased by adjusting switches Q1, Q2, Q3, Q4. Acontroller (not shown), suitably connected to a power supply, controlsthe operation of the switches Q1, Q2, Q3, Q4. In one embodiment, thecontroller can be off the shelf and possibly modified. In oneembodiment, the controller can have analog circuitry. In one embodiment,the controller can be a microcontroller. In one embodiment, thecontroller could be a combination of these features. As discussed below,a phase shift can be created between the currents controlled by theswitches Q1, Q2, Q3, Q4. The inductor LR and the capacitor CR constitutean LC (or tank) circuit. The primary winding Tp of the transformer T iscoupled to the secondary winding Ts. Diode D1, diode D2, and capacitorCout constitute a Delon rectifier circuit. In a positive cycle, diode D1charges the upper capacitor of capacitor Cout. In a negative cycle,diode D2 charges the lower capacitor of the capacitor Cout. Vout iseffectively two times the voltage across the secondary winding Ts of thetransformer T.

The local management unit (1200) requires a reliable current limitbecause it is required to charge a large input capacitance reflectedfrom the inverter (203). The local management unit (1200) needs to allowoperation with low input and output capacitance, because reliabilitydoes not allow the use of aluminum capacitors due to their limited lifeexpectancy. In many instances aluminum may not be suitable for the localmanagement unit (1200) for reasons of reliability.

Efficiency of the novel topology of the local management unit (1200)should be higher than 96 percent at the range of 20 percent to 100percent load. The topology of the local management unit (1200) shouldallow direct control of input impedance for smooth MPPT control, andshould minimize the need for damping networks (i.e., snubbers) in orderto limit EMI emissions to improve reliability and maximize efficiency.Further, the transformer should be protected from saturation. Isolationvoltage must be higher than 2000V, and switching losses reduced (i.e.,zero current switching/zero voltage switching). No load condition is tobe defined during inverter turn on.

The local management unit (1200) achieves the aforementioned performancegoals. FIGS. 13 through 18 illustrate waveforms to show performance ofthe local management unit (1200) and the reduction of snub voltagetransients without resort to a snubber network in the local managementunit (1200). In FIG. 13, waveform 1302 shows the current through theprimary winding Tp of the transformer T and waveform 1304 shows thedrain voltage at the switch Q1 at the MPPT point. The waveform 1304shows ringing on the square wave for only approximately two and a halfwaves at approximately one volt peak-to-peak.

In FIG. 14, waveform 1402 shows the current through the primary windingTp of the transformer T and waveform 1404 shows the drain voltage at theswitch Q1 at 30 percent load.

FIG. 15 shows low input voltage at full load condition. In FIG. 15,waveform 1502 shows the current through the primary winding Tp of thetransformer T and waveform 1504 shows the drain voltage at the switch Q1at full load condition. Steps (1503) in the waveform 1502 result from aphase shift between switches. The steps (1503) results is reducedundershoot and overshoot in the waveform 1504.

FIG. 16 shows output diode voltage at resonant frequency at maximumload. In FIG. 16, waveform 1602 shows the output current from the localmanagement unit (1200) to the inverter (203) and waveform 1604 showsdiode D1 (or diode D2) voltage at minimum frequency.

FIG. 17 shows typical output diode voltages at medium loads. In FIG. 17,waveform 1702 shows the output current from the local management unit(1200) to the inverter (203) and waveform 1704 shows diode D1 (or diodeD2) voltage at minimum frequency.

For loads higher than 15 percent of the maximum load, switches Q1, Q3are operated together at 50 percent duty cycle, while switches Q4, Q2are operated together at 50 percent duty cycle with no phase shift.Input power is controlled by changing operating frequency of the localmanagement unit (1200) above and below the resonant frequency. Turnratio of the primary winding Tp and secondary winding Ts is setaccording to MPPT voltage because at this voltage efficiency is at thehighest point (i.e., zero voltage, zero current is achieved). For otherfrequencies, switching is performed at zero voltage because there iscurrent in the primary winding Tp and resonant tank that is maintained,and this current causes voltage shift that allows turn-on to beperformed at zero voltage.

Below 15 percent of load, the local management unit (1200) is operatedin phase shift mode. In phase shift mode, switches Q1, Q2 are reversed,and switches Q3, Q4 are reversed. However, a phase shift causes switchesQ3 and Q4 to conduct together part of the time, and likewise forswitches Q1, Q4. A phase shift operation allows no load and light loadcontrol. As shown in, for example, FIG. 15, steps 1503 in the waveform1502 are caused by the phase shift. The phase shift range and frequencyrange are optimized for maximum efficiency by the local management unit(1200). The switches (primary transistors) do not have off spike becausethey are clamped to the input bus. The phase shift minimizes ringing(and overshoot and undershoot), which in turn increases efficiency,reduces EMI, and reduces heat losses. Secondary diodes D1, D2 areconnected in center tap configuration to prevent voltage spikes fromdeveloping across them during turn-off and eliminating need for clampingcomponents.

As shown in FIG. 16, a phase shift between the switches, as describedabove, causes a reduction in undershoot and overshoot in the diode D1voltage without implementation of snubber networks. As a result,efficiency of the local management unit (1200) is improved both on theswitch side and the diode side. In one embodiment, efficiency isimproved on each side by approximately 1-2%.

In the local management unit (1200), a resonant tank provides a limit tothe current through the primary winding Tp. A serial capacitor CRprevents transformer saturation. Output rectifier voltage is clamped tooutput voltage Vout allowing the use of 600V ultra fast diodes. Thereare no spikes across the switching transistors. Transformer parametersact as part of resonant tank. Input voltage range and efficiency areoptimized for solar module operation by transformer turn ratio andtransformer small air gap. Resonant frequency controls input impedance,which is the required control parameter for the application of separatesolar modules operating against a fixed voltage inverter load in thesystem.

FIG. 18 shows a spectral waveform (1802) of typical emissioncharacteristics of the local management unit (1200). Current ripple ofthe local management unit (1200) is measured with a current probe. Mostof the current ripple comes from the inverter (203). In one embodiment,the inverter (203) is an off the shelf item. From the spectral waveform(1802), it can be seen that data transmission is possible but needs tobe in the same level or higher level than the noise level. It can beseen that the maximum noise level value is approximately 35 dB.Switching frequency is clearly seen and can be detected in the spectralwaveform (1802).

FIG. 19 shows a local management unit (1900) that can be used inaccordance with the present invention. The local management unit (1900)can be used in place of the local management units discussed above. Thelocal management unit (1900) includes a capacitor C1, switches Q1, Q2,diode D1, inductor L, capacitor C2, controller 1902, terminals 1904,1906, 1908, and communication transmission modulator 1910. Operation ofthe local management unit (1900) is similar to the operation of thelocal management units, as discussed above. Data transmission by thelocal management unit (1900) involves modulating the switching frequencyof the local management unit (1900) and transferring data by using thesolar module itself as power amplifier (PA).

Operation of the local management units in FIGS. 1-3B and FIG. 12involve pulse width modulation (PWM), as discussed above. The PWMtechnique creates noise, as shown in, for example, FIG. 18. The creatednoise can be modulated to transmit data over the string bus (205) from asolar module (or slave node) to a head unit (master) in the energyproduction or photovoltaic system. The use of noise in this way avoidsthe need to provide a costly separate, dedicated communications channelfrom the solar module to the head unit.

The communication transmission modulator (1910) modulates switching ofthe pulse width modulation (PWM) operation to transmit data from thelocal management unit (1900). Various modulation encoding schemes can beused, such as, for example, modified FM (MFM) and Manchester coding. Inone embodiment, another modulating and encoding scheme can be used. Inone embodiment, the communication transmission modulator (1910)represents the transmission portion of a modem (not shown) that isassociated with the local management unit (1900). In one embodiment, thecommunication transmission modulator (1910) is part of the localmanagement unit (1900). In one embodiment, the communicationtransmission modulator (1910) is external to the local management unit(1900).

This system allows the use of full duplex (two-way) communications. Thereceiver at the module side can be implemented within the modulecircuitry. The limitation of transmit and receive within same circuitdoes not exist. Transmission from management unit can be used tosynchronize modules. Reliability is not affected by transmission. Theeffect on overall performance is very small because the transmissionduty cycle from the module is low.

Weak solar modules in a string bus, and weak string buses in a solararray can bring down the total output power of the solar array.Traditional solar arrays may not be able to overcome this problem sincethe output of individual solar modules and string buses may not becontrollable independently of other solar modules and string buses. Thesystems and methods herein disclosed monitor and adjust individual solarmodule outputs such that weak solar modules are balanced with strongsolar modules in a string bus, and strong string buses are balanced withweak string buses in a solar array. Once balancing within string busesand between string buses has been accomplished, an inverter usingmaximum power point tracking (MPPT) can determine the maximum powerpoint for the solar array.

Three I-V curves for a string bus will now be used to describe balancingsolar modules on a string bus. FIG. 27 illustrates an I-V curve (2700)for a string bus where all solar modules (2702) are operating at theirideal outputs. There are six solar modules (2702) in the illustratedstring bus. A string bus I-V curve (2704) is derived from a composite ofall six ideal solar module I-V curves (2702). The voltage of the stringbus (2704) is derived by adding the voltages provided by each solarmodule (2702). The MPP (2706) is the point on the string bus I-V curve(2704) where current times voltage is maximized.

FIG. 28 illustrates an I-V curve (2800) for a string bus where two solarmodules are operating as weak solar modules. The weak solar modules(2808) produce the same voltage as the strong solar modules (2802), butcannot produce as large of currents. The string bus sees a loss ofcurrent (2812), since the string bus (2804) cannot produce the maximumcurrent of the strong solar modules (2802). The MPP (2806) is lower thanthat seen in the ideal string bus illustrated in FIG. 27. In thisoperating region, the strong solar modules (2902) are operating atmaximum voltage, but are incapable of operating at their maximumcurrent. The string bus is thus producing less energy than what it iscapable of.

FIG. 29 illustrates an I-V curve (2900) for a string bus implementingthe systems and methods of this disclosure. The voltage of the two weaksolar modules (2908) can be decreased, which in turn increases theircurrent. The voltage can be decreased until the current balances withthe current from the strong solar modules (2902). This is what is meantby balancing solar modules on a string bus. As a result, the string busvoltage output decreases by a small value (2912) equal to the decreasein voltage for the two weak solar modules (2908). However, the stringbus current rises by a larger value (2914) to the level of what thestrong solar modules (2902) are capable of producing. The maximum powerproduced from the MPP (2906) of this I-V curve (2900) is greater thanthe maximum power produced when the weak solar modules are holding downthe current of the strong solar modules (FIG. 28). Stated differently,the sacrifice in voltage is more than compensated for by the increasedcurrent.

Three I-V curves for a solar array will now be used to describebalancing string buses in a solar array. FIG. 30 illustrates an I-Vcurve (3000) for a solar array where all string buses (3002) areoperating at their ideal outputs. There are nine string buses (3002) inthe illustrated solar array. A solar array I-V curve (3004) is derivedfrom a composite of all nine ideal string bus I-V curves (3002). Thecurrent of the solar array (3004) is derived by adding the currentsprovided by each string bus (3002). The MPP (3006) is the point on thesolar array I-V curve (3004) where current times voltage is maximized.

FIG. 31 illustrates an I-V curve (3100) for a solar array where twostring buses are operating as weak string buses. The weak string buses(3108, 3110) produce the same current as the strong string buses (3102),but cannot produce as large of voltages. The solar array sees a loss ofvoltage (3112), since the solar array (3104) cannot produce the maximumvoltage of the strong string buses (3102). The MPP (3106) is lower thanthat seen in the ideal solar array illustrated in FIG. 30. In thisoperating region, the strong string buses (3102) are operating atmaximum current, but are incapable of operating at their maximumvoltage. The solar array is thus producing less energy than what it iscapable of.

FIG. 32 illustrates an I-V curve (3200) for a solar array implementingthe systems and methods of this disclosure. Voltages of strong stringbuses (3202) can be decreased, which in turn increases the currentoutput from strong string buses (3202). The voltages can be decreaseduntil all string bus currents balance with the current from the weakeststring bus (3208). This is what is meant by balancing string buses in asolar array. As a result, the solar array voltage output decreases by avalue (3112) equal to the decrease in voltage for the strong stringbuses (3202). However, the solar array gain in current (3214) is equalto the sum of the increased currents from all of the strong string buses(3202) whose voltages were decreased. The maximum power produced fromthe MPP (3206) of this I-V curve (3200) is greater than the maximumpower produced when the weak string buses (3208) were holding down thevoltage of the strong string buses (3202). Stated differently, thesacrifice in voltage is more than compensated for by the increasedcurrent, even when conversion losses are accounted for.

In an embodiment, the voltages of the strong string buses (3202) can bedecreased until the current balances with one or more weak string buses(3208, 3210). Since the weak string buses (3208, 3210) may not operateat the same maximum voltage (e.g., (3210)>(3208)), strong string buses(3202) may be balanced with an average of the weak string buses (3208),(3210).

It should be understood that FIGS. 27-32 are not drawn to scale, and areillustrative only. The curvatures of the I-V curves are also merelyillustrative and can vary significantly depending on the actual systemsthat the I-V curves represent. Although six solar modules and ninestring buses were described, these numbers are illustrative only. Anynumber of solar modules and string buses can be used.

Balancing solar modules in a string bus means that the solar modulecurrents converge. In an embodiment, balancing solar modules in a stringbus means that weak solar module currents converge on strong solarmodule currents. Balancing string buses in a solar array means thatstring bus voltages converge. In an embodiment, balancing string busesin a solar array means that strong string bus voltages converge on weakstring bus voltages. It should be understood that balancing need notmean an exact balance. Two values may only converge to within athreshold value of each other—a relative equivalency. For instance, twocurrents that are to be balanced by causing them to converge on 1 amp,may be considered balanced if they come within 0.1 amp of the 1 ampgoal. Two currents that are to be balanced by causing them to convergemay be considered balanced when they are within five percent of eachother. However, even when balanced, the process is iterative and willcontinue indefinitely. This is because the solar module performancechanges, the load changes, and local conditions (e.g., clouds, leaves,dirt, to name a few). Furthermore, every balancing of a string bus mayrequire a balancing of solar modules on each string bus, and everybalancing of solar modules on each string bus may require a balancing ofstring buses.

FIG. 4B illustrates an embodiment of a solar array along with aninverter and a string combiner. In the illustrated embodiment the solararray 200 includes three string buses (205 a, 205 b, 205 c), althoughone or more string buses (205 a, 205 b, 205 c) can also be used. Thestring buses enable a series connection of solar modules (201 a, 201 b,. . . , 201 n). Coupled between each solar module (201 a, 201 b, . . .201 n) and its corresponding string bus (205 a, 205 b, 205 c), is alocal management unit (LMU) (202 a, 202 b, . . . , 202 n). The LMU's(202 a, 202 b, . . . , 202 n) are controlled by a controller (204). Thecontroller (204) can communicate wirelessly with the LMU's (202 a, 202b, . . . , 202 n) or via wireless repeaters. In an embodiment (notillustrated), wired connections between the controller (204) and theLMU's (202 a, 202 b, . . . , 202 n) can be implemented. String bus (205a, 205 b, 205 c) outputs are connected at an inverter (203) or in anoptional string combiner (206). The controller (204) may be configuredto balance current outputs from the solar modules on a string bus (205a, 205 b, 205 c). This can be done for each string bus (205 a, 205 b,205 c). Once, the current outputs from solar modules (201 a, 201 b, . .. , 201 n) on a string bus (205 a, 205 b, 205 c) are balanced (weaksolar module currents are raised to the level of strong solar modulecurrents within a string), the controller (204) may balance the currentoutputs from the string buses (205 a, 205 b, 205 c) (strong string busvoltages are lowered to the level of weak string bus voltages, which inturn raise strong string bus currents and hence the solar arraycurrent). This process can be repeated or an inverter (203) can thenattempt to determine the MPPT for the solar array (200).

A “solar array” typically comprises two or more solar modulesseries-connected via a string bus where the output voltage is a sum ofthe voltages of the series-connected solar modules. In larger solararrays, string buses can be connected in parallel such that theircurrents add. A combiner and inverter are not part of the solar array.

Balancing current outputs of solar modules (201 a, 201 b, . . . , 201 n)on a string bus (205 a, 205 b, 205 c) will now be discussed in moredepth. The controller (204) may be configured to balance the currentsproduced by the solar modules (201 a, 201 b, 201 n) on a given stringbus (205 a, 205 b, 205 c), and perform this balancing for each stringbus (205 a, 205 b, 205 c). As a result, the currents from the solarmodules (201 a, 201 b, . . . , 201 n) on a string bus (205 a, 205 b, 205c) may be balanced.

In order to balance solar modules (201 a, 201 b, . . . , 201 n) on astring bus (205 a, 205 b, 205 c), it can be useful to identify strongsolar modules (201 a, 201 b, 201 n) and weak solar modules (201 a, 201b, . . . , 201 n). This is done by varying the current on a string bus(205 a, 205 b, 205 c), monitoring the resulting change in voltage ineach solar module (201 a, 201 b, . . . , 201 n), and comparing thechanges in voltage on each solar module (201 a, 201 b, . . . , 201 n) toidentify strong solar modules (201 a, 201 b, . . . , 201 n) and weaksolar modules (201 a, 201 b, . . . , 201 n).

Varying the current on the string bus (205 a, 205 b, 205 c) may involvethe inverter (203) pulling a different current from the string bus (205a, 205 b, 205 c). It may involve varying an impedance seen by the stringbus (205 a, 205 b, 205 c). For instance, the inverter (203) can vary theimpedance that the string bus (205 a, 205 b, 205 c) sees, and in doingso the current and voltage produced by the solar modules (201 a, 201 b,. . . , 201 n) on the string bus (205 a, 205 b, 205 c) will vary. Inother words, pulling a different current or changing the impedancechanges where on the I-V curve each solar module (201 a, 201 b, . . . ,201 n) operates at. Since current for devices connected in series is thesame, a change in current on the string bus (205 a, 205 b, 205 c) causesthe same change in current for each solar module (201 a, 201 b, . . . ,201 n) on the string bus (205 a, 205 b, 205 c). However, the changes involtage may not be the same, since the solar modules (201 a, 201 b, . .. , 201 n) may operate at different operating points on the I-V curve.

This can be seen in FIG. 22, which illustrates an example of a compositeI-V curve (2203) for solar modules on a string bus. This composite I-Vcurve (2203) is not drawn to scale. Working points for two differentsolar modules can be seen in FIG. 22. The working point (2202) has alower-angled slope and represents a weak solar module. The working point(2201) has a higher-angled slope and represents a strong solar module.The variation in string bus current (2204 a) for the weak solar moduleis the same as the variation in string bus current (2204 b) for thestrong solar module since the solar modules are connected in series, andthus must operate at the same current. However, since the two solarmodules are at different working points on the I-V curve (2203), theresulting change in voltage (2206, 2205) for each is not the same. Thechange in voltage dV2 (2206) for the weak solar module is greater thanthe change in voltage dV1 (2205) for the strong solar module.

By identifying strong and weak solar modules, based on the changes involtage dV1, dV2, one can determine which solar module(s) (201 a, 201 b,. . . , 201 n) to adjust. Strong solar modules (201 a, 201 b, . . . ,201 n) can be used as a reference. Strong solar modules (201 a, 201 b, .. . , 201 n) may not be adjusted, while weak solar module (201 a, 201 b,. . . , 201 n) voltages can be decreased until their current outputsconverge on the strong solar module (201 a, 201 b, . . . , 201 n)outputs (or an average strong solar module (201 a, 201 b, . . . , 201 n)current output). This raises the current output of the string bus (205a, 205 b, 205 c), while decreasing the string bus (205 a, 205 b, 205 c)voltage output. However, the net effect is greater power output from thestring bus since the loss in voltage is more than compensated for by theincreased current. The end result may preferably be working points thatare proximal for all solar modules (201 a, 201 b, . . . , 201 n), thatis balanced or near balanced current outputs. An indication thatbalancing has been achieved and that the solar modules (201 a, 201 b, .. . , 201 n) are operating near the maximum current output of the strongsolar modules (201 a, 201 b, . . . , 201 n), is that a variation in thecurrent along the string bus (205 a, 205 b, 205 c) will cause a nearlyequivalent change in voltage for each solar module (201 a, 201 b, . . ., 201 n).

In an embodiment, instead of using strong solar modules (201 a, 201 b,201 n) as the reference, an average of all solar modules (201 a, 201 b,. . . , 201 n) on a string bus (205 a, 205 b, 205 c) can be used as areference. In this embodiment, all solar modules (201 a, 201 b, . . . ,201 n) on the string bus (205 a, 205 b, 205 c) can be adjusted,including strong solar modules (201 a, 201 b, . . . , 201 n), untiltheir current outputs converge on the average. In an embodiment, thecontroller (204) can identify strong and weak solar modules (201 a, 201b, . . . , 201 n) of all solar modules (201 a, 201 b, . . . , 201 n) ona string bus (205 a, 205 b, 205 c).

In an embodiment, the change in voltage dVi for each solar module (201a, 201 b, . . . , 201 n) can be checked for anomalies, and thosemeasurements appearing to be erroneous can be ignored or eliminated andreplaced with a new measurement of dVi. In an embodiment, at least onesolar module (201 a, 201 b, . . . , 201 n) can be identified as a strongsolar module (201 a, 201 b, . . . , 201 n). In an embodiment, the solarmodules (201 a, 201 b, . . . , 201 n) identified as strong solar modules(201 a, 201 b, . . . , 201 n) may be left out of the other stepsinvolved in balancing a string bus (205 a, 205 b, 205 c) (e.g., strongsolar module (201 a, 201 b, . . . , 201 n) current outputs may not bechanged while the current outputs of weak solar modules (201 a, 201 b, .. . , 201 n) are changed).

String buses (205 a, 205 b, 205 c) may be connected in parallel and havean output that is optionally connected to a string combiner (206) (orfuse box or chocks box). In an embodiment, the output of the stringbuses (205 a, 205 b, 205 c) can be connected to the inverter (203).

FIG. 21 illustrates exemplary solar module currents for strong and weaksolar modules. Peaks and troughs are caused by changes in current on astring bus that the two solar modules are coupled to. Since the twosolar modules may not operate at the same working point on the I-Vcurve, the resulting changes in voltage dV1 (2101) and dV2 (2102) maynot be the same. Here, the stronger solar module has a smaller dV1(2101) since its working point corresponds to higher voltage and lowercurrent. The weaker solar module has a larger dV2 (2102) since itsworking point corresponds to lower voltage and higher current. Bymonitoring these voltage fluctuations, a controller or LMU can decreasethe voltage, and increase the current of the weak solar modules in orderto shift dV2 towards dV1.

In an embodiment, the LMU's (202 a, 202 b, . . . , 202 n) control thevoltage and current provided to the string buses (205 a, 205 b, 205 c)from the solar modules (201 a, 201 b, . . . 201 n). In an embodiment,one LMU may control the voltage and current output for more than onesolar module (201 a, 201 b, . . . , 201 n). In an embodiment, the numberof solar modules (201 a, 201 b, . . . , 201 n) may exceed the number ofLMU's (202 a, 202 b, . . . , 202 n). For instance, LMU's (202 a, 202 b,. . . , 202 n) may only be used to control the current and voltageoutput from solar modules (201 a, 201 b, . . . , 201 n) identified asweak solar modules.

In an embodiment, a controller (204) may control the LMU's (202 a, 202b, 202 n). The controller (204) may also monitor the string buses (205a, 205 b, 205 c) and the solar modules (201 a, 201 b, . . . 201 n) viathe LMU's (202 a, 202 b, . . . , 202 n). Data regarding the solarmodules (201 a, 201 b, . . . 201 n) and LMU's (202 a, 202 b, . . . , 202n) can be transmitted via the string buses (205 a, 205 b, 205 c) to thecontroller (204). In an embodiment, the controller (204) can transmitinstructions or commands to the LMU's (202 a, 202 b, . . . , 202 n) viathe string buses (205 a, 205 b, 205 c). In another embodiment, thecontroller (204) can perform the above-noted communications with theLMU's (202 a, 202 b, . . . , 202 n) via wireless communication paths.The controller (204) may also be in communication with the inverter(203). In the illustrated embodiment, the controller (204) is astandalone device. However, in other embodiments, the controller (204)may be a part of other devices (e.g., the inverter (203), or LMU's (202a, 202 b, . . . , 202 n). In an embodiment, the controller (204) can bea part of one of the LMU's (202 a, 202 b, . . . , 202 n).

In an embodiment, operation of the controller (204) can be based onhistorical current and voltage data to help in pattern identification.For example, where the controller (204) notices that certain solarmodules (201 a, 201 b, . . . , 201 n) become weak solar modules (201 a,201 b, . . . , 201 n) at a specified time every day, this may be anindication of an object casting a predictable shadow over those solarmodules (201 a, 201 b, . . . , 201 n). As a result, instructions can besent to the affected LMU's (202 a, 202 b, . . . , 202 n) at the timewhen those LMU's (202 a, 202 b, . . . , 202 n) regularly become weak.

Balancing current outputs of string buses (205 a, 205 b, 205 c) will nowbe discussed in more depth. Although solar module (201 a, 201 b, . . .201 n) output current can be balanced on each string bus as describedabove, each string bus (205 a, 205 b, 205 c) may produce differentvoltages (i.e., weak string buses may produce less-than-ideal orless-than-maximum voltages).

Since string buses (205 a, 205 b, 205 c) may be connected in parallel,the voltages produced by the string buses (205 a, 205 b, 205 c) mayconverge. This voltage convergence causes the working points of thesolar modules (201 a, 201 b, . . . 201 n) in each string bus (205 a, 205b, 205 c) to change. FIG. 23 shows exemplary plots of the resultingchange in current seen on two string buses when connected in parallel.The stronger string sees a decrease in current while the weak stringsees an increase in current.

In an embodiment, varying the voltage of the string buses in the solararray involves varying the current drawn from the string buses (205 a,205 b, 205 c) or varying an impedance seen by the string buses (205 a,205 b, 205 c). For instance, an inverter (203) connected to the stringbuses (205 a, 205 b, 205 c) can vary the impedance that the string buses(205 a, 205 b, 205 c) see, and in doing so the current and voltageproduced by the solar modules (201 a, 201 b, . . . 201 n) on the stringbus (205 a, 205 b, 205 c) will change. In other words, changing theimpedance changes where on the I-V curve each string bus (205 a, 205 b,205 c), and the solar modules (201 a, 201 b, . . . 201 n) on each stringbus, (205 a, 205 b, 205 c) operate at. A change in voltage on the stringbuses (205 a, 205 b, 205 c) causes a change in the current output fromeach of the string buses (205 a, 205 b, 205 c). However, since thestring buses (205 a, 205 b, 205 c) may not operate at the same operatingpoint on the I-V curve, the change in voltage will cause differingchanges in current for some or all of the string buses (205 a, 205 b,205 c).

This can be seen in FIG. 25, which illustrates a composite I-V curve(2510) for string buses in the solar array (200). The I-V curve is notdrawn to scale. An operating point for two different string buses can beseen in FIG. 25. Working point (2508) represents a weak string bus andworking point (2507) represent a strong string bus. The variation instring bus voltage dV1 (2512) is the same variation as seen for stringbus voltage variation dV2 (2511) since the string buses are connected inparallel. However, since the two string buses are at different workingpoints on the I-V curve (2510), the resulting change in current dI1(2503) and dI2, (2504) are not the same. The change in voltage dI2(2503) for the strong string bus is smaller than the change in voltagedI1 (2504) for the weak string bus.

By comparing the change in currents dI1, dI2 one can determine whichstring buses (205 a, 205 b, 205 c) to adjust. Adjusting string busvoltage output involves equally decreasing the voltage output of allsolar modules on a string bus (205 a, 205 b, 205 c), resulting in anincrease of the current from the string bus (205 a, 205 b, 205 c). In anembodiment, one or more weak string buses (205 a, 205 b, 205 c) can beused as references such that all other string bus voltages are balancedwith that of the one or more weak string buses (205 a, 205 b, 205 c). Inanother embodiment, an average of weak string buses (205 a, 205 b, 205c) can be used as the reference. In an embodiment, an average of allstring buses (205 a, 205 b, 205 c) can be used as the reference.

Not all solar modules may be adjusted. For instance, if two or morestring buses (205 a, 205 b, 205 c) are producing about the same current,then those string buses (205 a, 205 b, 205 c) can be used as references(the outputs from solar modules (201 a, 201 b, . . . 201 n) on thosestring buses (205 a, 205 b, 205 c) will not be changed). Solar moduleoutput currents on all other string buses (205 a, 205 b, 205 c) can besuch that the string bus output currents converge on the output from thereference string buses (205 a, 205 b, 205 c).

Having identified weak string buses (205 a, 205 b, 205 c), an averagechange in current dIw for the weak string buses (205 a, 205 b, 205 c)can be determined. The average change in current dIw for the weak stringbuses (205 a, 205 b, 205 c) can be a reference value. The change incurrent dIi for each strong string bus (205 a, 205 b, 205 c) can becompared to the average change in current dIw for the weak string buses(205 a, 205 b, 205 c). The difference between dIi for a string bus (205a, 205 b, 205 c) and dIw indicates by how much the string bus (205 a,205 b, 205 c) output current should be decreased in order to match theoutput current of the weak string buses (205 a, 205 b, 205 c) (to pushthe strong string bus (205 a, 205 b, 205 c) working point (2507) towardsthe weak string bus (205 a, 205 b, 205 c) working point (2508).

For instance, and referring to FIG. 25, the weak string bus (205 a, 205b, 205 c) change in current (2503) dI2 is greater than the strong stringbus (205 a, 205 b, 205 c) change in current (2504) dI1. Thus, the strongstring bus (205 a, 205 b, 205 c) current could be increased, which wouldreduce the voltage. The string bus (205 a, 205 b, 205 c) working point(2508) for the strong string bus (205 a, 205 b, 205 c) would thus shifttowards the working point (2507) of the weak string bus (205 a, 205 b,205 c). Similarly, the change in current dIi for each string bus (205 a,205 b, 205 c) can be compared to the average change in current of theweak string buses dIw and the current outputs for the strong stringbuses (205 a, 205 b, 205 c) adjusted such that all the string buses (205a, 205 b, 205 c) in the solar array (200) have working points thatconverge on those of the weak string buses. The end result maypreferably be working points that are proximal for all string buses (205a, 205 b, 205 c)—balanced or near balanced current outputs. Anindication that balancing has been achieved and that the string buses(205 a, 205 b, 205 c) are operating at nearly identical working pointson the I-V curve, is that a variation in the voltage for all stringbuses (205 a, 205 b, 205 c) will cause a nearly equivalent change incurrent for each string bus (205 a, 205 b, 205 c).

In an embodiment, before weak string bus (205 a, 205 b, 205 c) outputsare adjusted, one or more weak string buses (205 a, 205 b, 205 c) can bedisconnected from the other string buses (205 a, 205 b, 205 c) todetermine if disconnecting the one or more weak string buses (205 a, 205b, 205 c) increases the power output from the solar array (200).

The inverter (203) can convert the direct current (DC) outputs from thestring buses (205 a, 205 b, 205 c) to an alternating current (AC) outputthat can be supplied, for example to a power grid or other load (e.g., ahome or business). The inverter (203) can control the current andvoltage drawn from the string buses (205 a, 205 b, 205 c), and thuscontrol where on the I-V curve the string buses (205 a, 205 b, 205 c)operate at. For instance, as the inverter (203) can increase impedanceseen by the solar array (200), which will cause the current drawn fromthe string buses (205 a, 205 b, 205 c) to decrease and the voltage toincrease, and the working point will shift along the I-V curve to theright towards where the I-V curve meets the x-axis (voltage). Thus, ifthe string buses (205 a, 205 b, 205 c) are operating at a working pointhaving a higher current and lower voltage than the MPP for the solararray (200), then the inverter (203) may increase the impedance causingthe string bus (205 a, 205 b, 205 c) working points to shift towards theMPP. Balancing can be carried out via the methods described withreference to FIG. 26.

FIG. 20 illustrates an exemplary inverter current controlled by amaximum power point tracking algorithm. Such a current may occur when asolar array is connected to an MPPT-enabled inverter such as, but notlimited to, SB300 made by SMA America, Inc., and IG2000 made by FroniusUSA, LLC. A typical MPPT algorithm pulls and pushes the current (2003)in the solar array (for example, by increasing and decreasing theimpedance seen by the solar array) causing I to fluctuate into peakcurrent spikes (2001 a-n) and valley current spikes (2002 a-n). Thesefluctuations cause solar module voltage fluctuations. These fluctuationscan be used to determine the MPP for solar modules.

FIG. 24 illustrates an exemplary current versus time diagram for astronger and weaker string bus when the voltage to the string busesfluctuates. The stronger string bus has a smaller current change (2401)since the working point is closer to the closed-circuit current (wherethe I-V curve meets the Y-axis). The weaker string bus has a largercurrent change (2402) since the working point is closer to theopen-circuit voltage (where the I-V curve meets the X-axis).

FIG. 26 illustrates an embodiment of a method (2600) of maximizing thepower output of a solar array by (1) balancing current outputs of solarmodules, (2) balancing voltage outputs of string buses, and (3) applyingan MPPT algorithm to the solar array. The method (2600) may alsomaximize power by identifying strong and weak solar modules beforebalancing current outputs of solar modules and by identifying strong andweak string buses before balancing voltage outputs of string buses. Themethod (2600) can be carried out via an optional first identifyoperation (2602), a first balance operation (2604), an optional secondidentify operation (2612), a second balance operation (2614), and anapply operation (2620).

The order that balancing operations and the applying operation occur incan vary. For instance, currents produced by solar modules in each ofone or more string buses may be balanced in a first balance operation(2604). Voltages produced by the one or more string buses may then bebalanced in a second balance operation (2614). An MPPT algorithm maythen be applied to the solar array in an apply operation (2620). Inanother embodiment, the method (2600) may begin with the first balanceoperation (2604) followed by one or more loops of the apply operation(2620). The second balance operation (2614) may then occur followed byone or more further loops of the apply operation (2620). It should beunderstood that the first and second balance operations (2604, 2614) andthe apply operation (2620) can operate in any order or pattern, witheach operation repeating one or more times before another operationoperates. The first and second balance operation (2604, 2614) can pausebetween operations to allow the apply operation (2620) to repeatnumerous times. In an embodiment, the balance operations (2604, 2614)can run simultaneously with the apply operation (2620). In anembodiment, the applying an MPPT algorithm is not a part of the method(2600). Rather, the method (2600) may merely include balancing the solarmodules and the string buses.

As noted above, the method (2600) includes the balance currents producedby solar modules in each of one or more string buses operation (2604).The solar modules may be connected by a string bus of the solar array,for instance in series. In an embodiment, balancing involves (1) varyingthe current on the string bus, (2) monitoring changes in voltage outputfor each solar module on the string bus, (3) comparing the monitoredchanges in voltage output for the solar modules on the string bus, and(4) adjusting the current output of one or more of the solar modulessuch that the current outputs from all solar modules on the string busconverge (see FIG. 29).

In an embodiment, before the first balance operation (2604), strong andweak solar modules can be identified via a first identify operation(2602). The first identify operation (2602) identifies, for each stringbus, one or more strong solar modules and one or more weak solarmodules. Weak solar modules can be adjusted such that their outputsconverge on those of the strong solar modules.

In an embodiment, the first identify operation (2602) may be carried outby: (1) varying the current on the string bus (or impedance seen by thestring bus), (2) monitoring changes in voltage output for each solarmodule, and (3) comparing the changes in voltage output. Strong solarmodules may be those having the smallest change in voltage. Strong solarmodules may also be characterized as operating at relatively the samecurrent (indicated by similar changes in voltage). Strong solar modulesmay be those having working points furthest to the right on the I-Vcurve (e.g., working point (2201) in FIG. 22).

Having identified strong solar modules, an average change in voltage dVsfor the strong solar modules can be determined. The average change involtage dVs for the strong solar modules can be a reference value. Thechange in voltage dVi for each solar module can be compared to theaverage change in voltage dVs for the strong solar modules. Thedifference between dVi for a solar module and dVs indicates by how muchthe solar module output current should be decreased in order to matchthe output current of the strong solar modules (to push the weak solarmodule working point (2202) towards the working point (2201) of thestrong solar modules).

In an embodiment, once the first identify and balance operations (2602,2604) have been performed, strong and weak solar modules can again beidentified via the first identify operation (2602). This may be donesince some strong solar modules may have become weak while the weaksolar modules were being adjusted. If a weak solar module is identifiedthat was previously a strong solar module, then its current output maybe adjusted in the next loop of the first balance operation (2604).

The method (2600) may also include a second balance operation (2614). Inan embodiment, the second balance operation (2614) includes (1) varyingthe voltage of the string buses in the solar array, (2) monitoringchanges in current of each string bus, (3) comparing the monitoredchanges in current, and (4) adjusting the voltage output of the stringbuses (by changing the voltage output of all solar modules in a stringbus) such that the string bus output voltages converge (see FIG. 32).

In an embodiment, before the second balance operation (2614), strong andweak string buses can be identified via a second identify operation(2612). The second identify operation (2612) identifies strong and weakstring buses in the solar array. Strong string buses can be adjustedsuch that their outputs converge on those of the weak string buses. Inan embodiment, the second identify operation (2612) may be carried outby: (1) varying impedance seen by the string buses (or the voltage onthe string buses), (2) monitoring changes in current output for eachstring bus, and (3) comparing the changes in current output. Weak stringbuses may be those having the smallest change in current. Weak stringbuses may also be characterized as operating at relatively the samecurrent (indicated by similar changes in current). Weak string buses maybe those having working points furthest towards the left of the I-Vcurve (e.g., working point (2507) in FIG. 25).

The method (2600) also includes an apply operation (2620) wherein themaximum power point tracking algorithm is applied. Maximum power pointtracking (MPPT) is a procedure or algorithm used to determine themaximum power point (MPP) of a system—in this case the maximum powerpoint of a solar array (see, e.g., “Maximum power” in FIG. 22 andworking point (2506) in FIG. 25). In other words, an MPPT algorithm canadjust the current and voltage produced by the solar array until thevoltage times current is maximized. A variety of algorithms and devicescan be used to carry out MPPT. For instance, in an embodiment, aninverter connected to an output of the solar array can change theimpedance that the solar array sees, thus causing the voltage andcurrent produced by the solar array to change. By sweeping or flutteringthe impedance over one or more ranges of values, an MPPT algorithm candetermine what impedance corresponds with the MPP, and set the impedanceto that value such that the solar array operates at the MPP. In anotherembodiment, MPPT can include the steps of (1) adjusting the impedancethat the solar array sees (or adjusting the solar array voltage orcausing the working point to move along the I-V curve), (2) monitoringthe solar array's reaction to the adjusting impedance (or current orworking point), (3) continuing to adjust the impedance (or current orworking point) and monitoring the solar array's response, (4) based onthe monitoring, determine a maximum power point for the solar array, and(5) set the impedance (or current or working point) to a value thatcorresponds to the solar array's MPP.

The apply operation (2620) can operate after either of the balanceoperations (2604, 2614). The apply operation (2620) can also operateafter any number of repetitions or loops of either or both of thebalance operations (2604, 2614). In an embodiment, the apply operation2620 may operate after any number of repetitions or loops of the firstidentify operation (2602) and the first balance operation (2604); afterany number of repetitions or loops of the second identify operation(2612) and the second balance operation (2614); and after any number ofrepetitions or loops of a combination of the first and second identifyand apply operations (2602, 2604, 2612, 2614). For instance, strong andweak solar modules in each of one or more string buses can be identifiedin the first identify operation (2602). Currents produced by solarmodules in each of the one or more string buses may be balanced in thefirst balance operation (2604). Strong and weak string buses in each ofthe one or more string buses can be identified in the second identifyoperation (2612). Voltages produced by the one or more string buses maythen be balanced in the second balance operation (2614). An MPPTalgorithm may then be applied to the solar array in the apply operation(2620).

In another embodiment, the method (2600) may begin with the firstidentify operation (2602) and the first balance operation (2604)followed by one or more loops of the apply operation (2620). The secondidentify operation (2612) and the second balance operation (2614) maythen occur followed by one or more further loops of the apply operation(2620). It should be understood that the first and second identifyoperation (2602, 2612), the first and second balance operations (2604,2614), and the apply operation (2620) can operate in any order orpattern, with each operation repeating one or more times. In anembodiment, the apply operation (2620) can operate multiple times beforethe first or second identify operations (2602, 2612) resume.Alternatively, the identify and balance operations (2602, 2604, 2612,2614) can run simultaneously with the apply operation (2620). In anembodiment, the applying an MPPT algorithm is not a necessary part ofthe method (2600). Rather, the method (2600) may merely includebalancing the solar modules and the string buses.

It is clear that many modifications and variations of this embodimentmay be made by one skilled in the art without departing from the spiritof the novel art of this disclosure. For example, the systems and methodherein disclosed may be applied to energy generating systems besidessolar photovoltaics (e.g., windmills, water turbines, hydrogen fuelcells). These modifications and variations do not depart from thebroader spirit and scope of the invention, and the examples cited hereare to be regarded in an illustrative rather than a restrictive sense.

In the foregoing specification, the disclosure has been described withreference to specific exemplary embodiments thereof. It will be evidentthat various modifications may be made thereto without departing fromthe broader spirit and scope as set forth in the following claims. Thespecification and drawings are, accordingly, to be regarded in anillustrative sense rather than a restrictive sense.

What is claimed is:
 1. A photovoltaic system, comprising: a plurality of photovoltaic modules; a plurality of management units coupled with the plurality of photovoltaic modules respectively, wherein each respective module in the photovoltaic modules generates electric power; a respective management unit in the plurality of management units is connected to the respective module to receive the electric power as an input and generate a power output from the input; and power outputs of the plurality of management units are connected in series; wherein the power outputs of the plurality of management units have respective maximum currents in current-voltage output characteristics of the power outputs of the plurality of management units; and wherein the plurality of management units are configured to match the respective maximum currents with each other by adjusting the current-voltage output characteristics of the power outputs of the plurality of management units.
 2. The photovoltaic system of claim 1, further comprising: a maximum power point tracker coupled to a series connection of the power outputs of the plurality of management units, the maximum power point tracker configured to track a maximum power point of the series connection as a whole.
 3. The photovoltaic system of claim 2, wherein the respective management unit includes a power converter configured to convert the input to the power output.
 4. The photovoltaic system of claim 3, wherein the respective management unit is configured to adjust operations of the power converter to adjust the current-voltage output characteristics of the power output of the respective management unit.
 5. The photovoltaic system of claim 4, wherein the operations of the power converter are adjusted based at least in part on a duty cycle of a switch of the power converter.
 6. The photovoltaic system of claim 5, wherein the respective management unit provides the electric power received from the respective module solely to the power output connected in the series connection.
 7. A photovoltaic system, comprising: a plurality of photovoltaic modules; a plurality of management units coupled with the plurality of photovoltaic modules respectively, wherein each respective module in the photovoltaic modules generates electric power; a respective management unit in the plurality of management units is connected to the respective module to receive the electric power as an input and generate a power output from the input; and power outputs of the plurality of management units are connected in parallel to a bus; wherein the power outputs of the plurality of management units have respective maximum voltages in current-voltage output characteristics of the power outputs of the plurality of management units; and wherein the plurality of management units are configured to match the respective maximum voltages with each other by adjusting the current-voltage output characteristics of the power outputs of the plurality of management units.
 8. The photovoltaic system of claim 7, further comprising: a maximum power point tracker coupled to a parallel connection of the power outputs of the plurality of management units, the maximum power point tracker configured to track a maximum power point of the parallel connection as a whole.
 9. The photovoltaic system of claim 8, wherein the respective management unit includes a power converter configured to convert the input to the power output.
 10. The photovoltaic system of claim 9, wherein the respective management unit is configured to adjust operations of the power converter to adjust the current-voltage output characteristics of the power output of the respective management unit.
 11. The photovoltaic system of claim 10, wherein the operations of the power converter are adjusted based at least in part on a duty cycle of a switch of the power converter.
 12. The photovoltaic system of claim 11, wherein the respective management unit provides the electric power received from the respective module solely to the power output connected in the parallel connection.
 13. The photovoltaic system of claim 7, wherein the respective module includes a plurality of solar modules.
 14. The photovoltaic system of claim 13, wherein outputs of the solar modules are connected in series in the respective module.
 15. A photovoltaic system, comprising: a plurality of photovoltaic modules; a plurality of management units coupled with the plurality of photovoltaic modules respectively, wherein each respective module in the photovoltaic modules generates electric power; and a respective management unit in the plurality of management units is connected to the respective module to receive the electric power as an input and generate a power output from the input; wherein the plurality of management units are partitioned into a plurality subsets; wherein power outputs of management units in each of the subsets are connected in series to provide a string output; wherein string outputs of the plurality of subsets are connected in parallel to a bus; wherein the power outputs of the plurality of management units have respective maximum currents in current-voltage output characteristics of the power outputs of the plurality of management units; wherein the string outputs of the plurality of subsets have respective maximum voltages in current-voltage output characteristics of the string outputs of the plurality of subsets; and wherein the plurality of management units are configured to, by adjusting power conversion operations of the plurality of management units: match, within each subset in the plurality of subsets, with each other the respective maximum currents of management units in the subset; and match with each other the respective maximum voltages of the string outputs.
 16. The photovoltaic system of claim 15, further comprising: a maximum power point tracker coupled to a bus to track a maximum power point of the plurality of photovoltaic modules as a whole.
 17. The photovoltaic system of claim 16, wherein the respective management unit includes a power converter configured to convert the input to the power output.
 18. The photovoltaic system of claim 17, wherein the respective management unit is configured to adjust operations of the power converter to adjust the current-voltage output characteristics of the power output of the respective management unit.
 19. The photovoltaic system of claim 18, wherein the operations of the power converter are adjusted based at least in part on a duty cycle of a switch of the power converter.
 20. The photovoltaic system of claim 18, wherein the power converter is a direct current to direct current converter. 